Edible Films and Coatings for Fresh Fish Packaging : Focus on Quality Changes and Shelf-life Extension

Fresh fish is extensively consumed and is one of the most-traded food commodities in the world. Conventional preservation technologies include vacuum and modified atmosphere packaging, but they are costly since requires capital investment. In the last decade, research has been directed towards the development of antimicrobial packaging systems, as an economical alternative to these. This paper outlines antimicrobial films and coatings applied so far on fresh fish, their efficacy against targeted microorganism/group and effects on chemical quality of the product. Findings show that edible films/coatings incorporated with different active agents applied to fresh fish are able to inhibit the microbial growth and decrease the rate of fish nutrients degradation, thus preventing the formation of chemical metabolites; a shelf-life extension of 6 to 13 days was obtained for fish fillets, depending on the species on which the active packaging materials were applied. The manufacturing use of these formulations could lead to a significant reduction in fish waste, consequently, a diminution of economic losses for fish traders and retailers. Therefore, their industrial production and commercialization could be an exploitable sector by the packaging industry.


Introduction
Fish is one of the most-traded food commodities worldwide [1].Capture fisheries and aquaculture provide valuable economic and social benefits to those who work in these industries [2].However, post-harvest handling, processing, and storage of fish lead to food losses and waste [3].Post-harvest losses occur at all stages in the fish supply chain from capture to consumer [4].The losses can be physical, economical, or nutritional and are caused by spoilage or poor processing [5].Spoilage is the process in which fish deteriorates to the point that becomes unacceptable for human consumption (with altered taste, smell, appearance, or texture) [6].Globally, fish losses that are caused by spoilage account for around 10% (10 to 12 million tons per year) of the total production from capture fisheries and aquaculture [7].
Fresh fish is a highly perishable product due to its high water activity, nutrient availability, nearly neutral-pH (factors that influence microbial growth) and the presence of autolytic enzymes; hence, it is susceptible to post-harvest losses [8,9].Under normal refrigerated storage conditions, its shelf-life is limited by the development of enzymatic (caused by endogenous or microbial enzymes) and chemical reactions [10].The main initial causative factor for fish spoilage is microbial growth and invasion, followed by the autolytic enzymes and then by chemical reactions, such as oxidation or hydrolysis [11,12].
Post-harvest losses of fresh fish due to microbial spoilage are a matter of great importance to the fishing industry [13].So, specific requirements and preservation techniques are needed to minimize the activity of spoilage bacteria.Fresh fish products are presently stored on ice or under refrigeration during their distribution and marketing.In these conditions, their shelf-life is limited to 5-10 days (depending on species, harvest location, and season) and they can result in enormous economic losses to fish traders and retailers [14,15].Therefore, the fish-process industry is actively seeking alternative methods of shelf-life preservation and marketability of fresh fish [16].
Packaging plays a critical role in the fish supply chain and is part of the solution to tackle food waste [17,18].Vacuum packaging (VP) and modified atmosphere packaging (MAP) are very commonly used as a supplement to ice or refrigeration to inhibit the normal spoilage flora and extend the shelf-life of fresh fish products [14,19,20].MAP technology has, however, some disadvantages, such as added costs for packaging equipment, gases, and packaging materials; it also requires special training for food operators [21].
Packaging innovation and new technologies is a necessity for the fishing industry.In recent years, a variety of active packaging systems have been developed to prolong storage life and enhance the safety of fish products.These have a variety of advantages such as biodegradability, edibility, biocompatibility, and aesthetic appearance, respectively, barrier properties against oxygen and physical stress [22].The purpose of this paper is to provide an overview of published research about edible films and coatings applied to fresh fish.The antimicrobial films and coatings that are used for fish packaging and their effects on chemical quality of fresh fish are reviewed and discussed (Figure 1).

Microbiological Issues
Fresh fish spoils due to the action of a group of microorganisms, the so-called specific spoilage organisms (SSOs).These organisms have the ability to dominate the fish flora and produce metabolites that directly affect the sensory properties of the product resulting in its rejection by consumers [23].During storage, the microflora changes owing to different capacities of the microorganisms to tolerate the preservation conditions [24].Under aerobic iced storage, the flora of fish is composed almost exclusively of Pseudomonas spp.and Shewanella putrefaciens (SSOs) regardless of whether it was caught or harvested in temperate or sub-tropical and tropical waters.At ambient temperature (25 • C), microflora is dominated by mesophilic Vibrionaceae, and, particularly if the fish is caught in polluted waters, by mesophilic Enterobacteriaceae [25].
Microbial spoilage is due to the proliferation of microorganisms after the death of fish as a result of the immune system collapsing, followed by the microbial invasion of the fish body through the skin [12].Fish have a unique osmoregulatory mechanism to avoid dehydration in marine environments and waterlogging of tissue in freshwater; it contains osmoregulatory compounds, like trimethylamine oxide (TMAO) and urea [26].Microbial enzymes that are present in fish can break down TMAO to trimethylamine (TMA) and urea to ammonia, volatile organic compounds associated with microbial spoilage [12].Many other volatile compounds can be formed by microbial enzymatic degradation of other substrates, such as hydrogen sulphide (from cysteine), methanethiol and methyl sulphide (from methionine), histamine (from histidine), acetate, carbon dioxide and water (from carbohydrates and lactate), hypoxanthine (from inosine and inosine-5 -monophosphate), esters, ketones, aldehydes (from amino acids, like glycine, serine, and leucine), as well as ammonia (from amino acids and urea) [12,26].These molecules are responsible for sweet, fruity, ammonia-like, putrid, and sulphuric off-flavours in spoiled fish [27].

Antimicrobial Films and Coatings Applied on Fresh Fish
This chapter provides an overview of previous research on the antimicrobial packaging of fresh fish.Table 1 lists active edible films and coatings applied to fresh fish fillets (of rainbow trout, silver carp, grass carp, beluga sturgeon, salmon, pike-perch, Japanese sea bass, red drum, golden pomfret, and hake) to extend its shelf-life.These films and coatings were produced from edible polymers like gelatin, chitosan, chitosan-gelatin, gelatin-alginate, carrageenan, quince seed mucilage, whey protein concentrate, and whey protein isolate incorporated with various active agents (essential oils (EOs) of clove, cinnamon, oregano, thyme, and lemon, glycerol monolaurate, α-tocopherol, lactoperoxidase, citric acid, licorice extract, grape seed extract, and tea polyphenols).Their antimicrobial efficacy was investigated in situ against spoilage and pathogenic microorganisms.Different levels of effectiveness were noticed, depending on the active agent used, its concentration, storage temperature, atmosphere composition (normal or modified), and targeted microorganism/group.

Efficacy against Tested
Microorganism/Group at the End of Monitoring Time
In a recent study, Carrión-Granda et al. (2018) [34] have examined the efficacy of coatings based on 10% whey protein isolate incorporated with different concentrations of oregano and thyme EOs (1% and 3%) under air and MAP conditions against Pseudomonas spp., H 2 S producing bacteria, and lactic acid bacteria in hake fillets.The application of coating with 1% thyme EO under MAP has shown the best results against Pseudomonas spp.but against H 2 S producing bacteria and lactic acid bacteria, the one with 3% oregano EO under the MAP.Different inhibitory effects displayed by an essential oil against various bacteria are most probably due to its chemical composition [35].The antimicrobial mechanism of action of plant EOs is related to the hydrophobicity of their components [36], which enables them to migrate in the lipids of the bacterial cell membrane and mitochondria, disturbing their structures and rendering them more permeable [37]; leakage of ions and intracellular constituents can thus occur [38].

Efficacy against Pathogenic Microorganisms
According to current literature, few studies on the efficacy of active packaging materials against pathogenic microorganisms in fresh fish have been published.Findings of such in situ investigations are presented in Table 1.Gómez-Estaca et al. ( 2009) [39] have tested the efficacy of edible films based on 8% gelatin and 8% gelatin/chitosan, both incorporated with 7.5% clove EO on salmon fillets, in vitro against Listeria innocua and Escherichia coli, then in situ against total viable organisms.The film based on gelatin was more effective against both bacteria than the one based on gelatin/chitosan; the ionic and hydrogen bonds that were formed between gelatin and chitosan diminished the solubility of the resulting film, thus reducing the amount of clove EO released.However, in the in situ experiment, they used the film based on gelatin/chitosan for storage trials.Their previous studies revealed that the low water solubility of the gelatin/chitosan matrix gives the film stability under fish contact conditions during chilled storage.
There are also some studies on fish fillets challenged with pathogenic bacteria.Han et al. (2013) [40] have investigated the efficacy of films based on 6.75% (w/w) gelatin, with and without nisin-incorporated, against Listeria monocytogenes in rainbow trout fillets that were challenged with 2 log CFU/g inoculum before and after coating.The edible film incorporated with 18 µg/cm 2 nisin, applied before inoculation, showed the highest inhibitory effect on Listeria monocytogenes.
The efficacy of gelatin coatings containing different concentrations of oregano EO (0.5%, 1.0%, and 2.0% v/v) was also investigated by Min and Oh (2009) [41], in catfish fillets that were inoculated with Salmonella typhimurium and Escherichia coli O157:H7.The coating based on 3% (w/v) gelatin containing 2% oregano EO exhibited the best inhibitory effect on both bacteria.
The most effective edible films/coatings against Enterobacteriaceae (including coliform bacteria) were those incorporated with 2% thyme EO [28], 1.5% oregano EO [29], and 1% lemon EO [30] that were applied on rainbow trout fillets.Edible coating with 8% whey protein concentrate/glycerol, 2:1 has also shown to be effective against Enterobacteriaceae in rainbow trout fillets as compared with the other formulations tested in the study [31].
When tested against total yeasts and moulds, the edible coating based on 0.4% chitosan and 3.6% gelatin applied to golden pomfret fillets was the most effective among all formulations [47].
In the work of Carrión-Granda et al. (2018) [34], the edible coating incorporated with 3% oregano EO was the most effective against total viable organisms, total psychrotrophic bacteria, as well as Enterobacteriaceae when applied under the MAP conditions.
The results of these investigations are not comparable, since, on the same fish species, were applied edible films/coatings with different polymer matrices, respectively active agents and evaluated in different storage conditions (temperature, atmosphere composition, and storage time).We noticed, however, some tendencies that allow us to affirm that:

•
edible films/coatings with the highest concentration of active agent tested have shown the greatest antimicrobial efficacy; • antimicrobial films/coatings were more effective at lower temperatures when tested in different storage temperature conditions; and, • under modified atmosphere packaging conditions, antimicrobial films/coatings were more effective than under air conditions.
Other authors have noticed that the effectiveness of antimicrobial packaging material depends also on the initial microbial load [40], chemical composition, and pH of tested food products [37].Generally, the susceptibility of bacteria to the antimicrobial effect of EOs is increased in products with low-fat content and low pH, respectively.

Efficacy of Edible Films/Coatings on Enhancing the Shelf-Life of Fresh Fish
The application of above-mentioned edible films and coatings to fish fillets resulted in an extension of their shelf-life as compared to uncoated controls.The film based on 1% quince seed mucilage incorporated with 2% thyme EO prolonged the shelf-life of rainbow trout fillets by 12 days [28] and the one based on 3% gelatin and 1.5% alginate incorporating 1.5% oregano EO by 6 days [29]; the coating based on 2% chitosan incorporated with 1.5% cinnamon EO by 8 days [42], the one based on 1% carrageenan incorporated with 1% lemon EO by 12 days [30], and the one based on 8% whey protein concentrate/glycerol, 2:1 by 6 days [31].In these cases, the shelf-life was stated considering a maximum acceptable level of 7.0 log CFU/g for the total viable count.
On beluga sturgeon fillets, the coating based on 8% whey protein concentrate incorporated with 1.5% cinnamon EO [45] extended the shelf-life by 12 days.
The study of Shokri & Ehsani (2017) [33] on pike-perch fillets show a shelf-life prolongation by 8 days when a packaging material based on 10% whey protein isolate incorporated with 2.5% lactoperoxidase was used for coating.
Another study, carried out by Qiu et al. ( 2014) [44], has shown an increased storage stability (from 8 to 12 days) of Japanese sea bass fillets coated with a solution containing 1.5% chitosan and 0.5% citric acid [44].
The coating formulation of Li et al. ( 2013) [43], also based on 1.5% chitosan but incorporated with 0.2% tea polyphenols, prolonged the microbiological shelf-life of red drum fillets by 8 days.
In a study on hake fillets, Carrión-Granda et al. (2018) [34] reported a shelf-life prolongation by 8 days when a coating based on 10% whey protein isolate incorporated with 3% oregano EO was used under MAP conditions.
Our review also revealed some studies in the existing literature focused on the application of synthetic films to fresh fish fillets.Cardoso et al. (2017) [50] have tested the efficiency of films based on poly(butylene adipate-co-terephthalate) incorporated with different levels of oregano EO (2.5%, 5.0%, 7.5%, and 10%) in lessening coliform bacteria, Staphylococcus aureus, and total psychrotrophic bacteria in fish fillets.The film incorporated with 10% (w/w) oregano EO showed the highest inhibitory effect on all bacteria leading to a shelf-life extension of 6 days for wrapped samples.The shelf-life was established considering a maximum acceptable level of 5.0 log CFU/g for Staphylococcus aureus.
In another study, Rollini et al. (2016) [51] have evaluated the efficacy of film based on polyethylene terephthalate coated with 3% (w/v) lysozyme and lactoferrin water solution, respectively, coextruded multilayer film based on polypropylene incorporated with 4.8% carvacrol against total mesophilic bacteria, total psychrotrophic bacteria, Enterobacteriaceae (including coliform bacteria), lactic acid bacteria, Pseudomonas spp., and H 2 S producing bacteria.The film that was coated with 3% lysozyme-lactoferrin has shown the best antibacterial results on total mesophilic bacteria, total psychrotrophic bacteria, lactic acid bacteria, and H 2 S producing bacteria, but the one incorporated with 4.8% carvacrol on Enterobacteriaceae (including coliform bacteria) and Pseudomonas spp.All of the samples were stored for up to four days; therefore, no extension of shelf-life was possible to notice for treated samples in such a short period of storage.
At high levels of incorporation with EOs, active films/coatings may impart foreign flavours to the products on which are applied.Of all the studies that are mentioned in Table 1, only two mentioned their effects on the sensory attributes of fresh fish.The study of Jouki et al. (2014) [28] revealed no significant negative effect of films based on 1% quince seed mucilage incorporated with oregano and thyme EOs in concentrations of up to 2% on the organoleptic acceptability of rainbow trout fillets.
Similar observations were also reported by Ojagh et al. (2010) [42] when a coating based on 2% chitosan incorporated with 1.5% cinnamon EO treatment was applied.

Effects of Edible Films/Coatings on the Chemical Quality of Fresh Fish
Table 2 summarizes the effects of the above-mentioned edible films and coatings on the chemical quality of fresh fish.Chemical indicators of lipid oxidation (TBARS-thiobarbituric acid reactive substances), degradation of nitrogen-containing compounds (TVB-N-total volatile basic nitrogen and TMA-N-trimethylamine nitrogen), and adenosine triphosphate breakdown (k-value) were measured during storage of fish fillets.
The thiobarbituric acid reactive substances (TBARS) assay is commonly used to evaluate malondialdehyde (MDA) content.MDA is one of the most significant products of lipid damage [52].Several researchers [28,33,42] have proposed maximum permitted levels for TBARS although the threshold criteria have not yet received regulatory approval; values <3 mg MDA/kg for perfect quality material, 3 ≤ MDA/kg < 5 for good quality material, and 5 ≤ MDA/kg < 8 for suitable for human consumption.In the published data reviewed in the current paper, TBARS values ranged from 0.2 to 0.9 mg MDA/kg for rainbow trout fillets, 3.0 to 4.0 mg MDA/kg for silver carp fillets, 0.9 to 1.2 mg MDA/kg for grass carp fillets, 0.06 to 0.12 mg MDA/kg for beluga sturgeon fillets, 1.1 to 1.8 mg MDA/kg for salmon fillets, 1.0 to 2.5 mg MDA/kg for pike-perch fillets, 0.2 to 2.0 mg MDA/kg for Japanese sea bass fillets, and 0.8 to 1.8 mg MDA/kg for red drum fillets; samples meeting the requirements for good quality material, respectively perfect quality material.
Most marine fish contain TMAO [57].TMAO is also found, with few exceptions, in freshwater fish, but only in small concentrations [58].Certain bacteria that occur naturally on the skin, in the guts of fish, and in water can break down TMAO to TMA.The amount of trimethylamine nitrogen (TMA-N) produced is a measure of the activity of spoilage bacteria in the flesh and so is an indicator of the degree of spoilage [57].There are no regulatory limits available for TMA level in fish.The rejection limit proposed by Jouki et al. (2014) [28] was <5 mg N/100 g and by Souza et al. (2010) [46] ≤5 mg N/100 g.K-value is an important chemical index widely used for fish freshness [59].During post-mortem storage of fish, autolytic changes take place in the muscle that determines adenosine triphosphate (ATP) degradation with the formation of adenosine-5 -diphosphate (ADP), adenosine-5 -monophosphate (AMP), inosine-5 -monophosphate (IMP), inosine (HxR), and hypoxanthine (Hx).K-value is calculated as the percentage of the sum of HxR and Hx, divided by the sum of ATP, ADP, AMP, IMP, HxR, and Hx [12,59].Since there are no legally enforceable limits for k-value in fish, Yu et al. (2017) [32] proposed the following freshness criteria: very fresh fish (k-value < 20%), moderately fresh (k-value < 60%), and spoiled (k-value > 60%).K-values reported in the discussed studies ranged from 68.7% to 90.5% for grass carp fillets, 46% to 50% for salmon fillets and 40% to 62.6% for red drum fillets; samples meeting freshness criteria for moderately fresh, respectively spoiled.

Conclusions
The active packaging of fish represents an economic alternative to conventional preservation technologies (vacuum and modified atmosphere packaging) due to the limited capital investment as compared to those.Besides being biodegradable, edible films and coatings improve the microbiological stability of fish and reduce waste; moreover, retard lipid oxidation.For the past 10 years, research on the use of antimicrobial packaging materials for fresh fish applications has undergone considerable evolution; nevertheless, as far as we know, there is not yet an edible film or coating commercially available on the market.
Fish represent one of the most-traded segments of the world food sector.Therefore, there is a great demand for the packaging of this good.Industrial production and commercialization of antimicrobial packaging materials for fresh fish could be an exploitable sector by the packaging industry.Suppliers of active packaging materials on the European market need to make sure that their products comply with the requirements of Regulations (EC) 1935/2004 [60] and (EC) 450/2009 [61] regarding active and intelligent materials that are intended to come into contact with food, respectively, Regulation (EC) 1333/2008 [62] that lays down specifications for food additives.Additional studies are however needed to further validate these findings, especially on the stability of antimicrobial films/coatings during shipment, storage, and handling.

Figure 1 .
Figure 1.Antimicrobial films and coatings used to extend the shelf-life of fresh fish fillets.

Table 1 .
Antimicrobial films and coatings used for packaging fish.

Table 2 .
Effects of antimicrobial packaging on chemical quality of fresh fish.